KIEV — With hundreds of riot police officers advancing from all sides after a day of deadly mayhem here in the Ukrainian capital, antigovernment protesters mounted a final desperate and seemingly doomed act of defiance late Tuesday evening, establishing a protective ring of fire around what remained of their all-but-conquered encampment on Independence Square.

Feeding the blazing defenses with blankets, tires, wood, sheets of plastic foam and anything else that might burn, the protesters hoped to prolong, for a while longer at least, a tumultuous protest movement against President Viktor F. Yanukovych, a leader who was democratically elected in 2010 but is widely reviled here as corrupt and authoritarian.